#include<stdio.h>

int main()
{
    int a[9] = {1,2,3,4,5,6,7,8,9};
    int b[9] = {9,8,7,6,5,4,3,2,1};

    int result[100] = {0};
    
    for(int i = 8;i >= 0 ; i--)
    {
        for(int j = 8; j>= 0 ; j--)
        {
            int num = a[i] * b[j];
            int sum = result[i+j+1] + num;
            result[i+j+1] = sum % 10;
            result[i+j] += sum / 10;

        }

    } 
  int flag = 1; 
 for(int i = 0;i<99;i++)
    { 
   
       if(result[i] == 0)    // 状态切换的时机： 碰到了第一个为 0 的数
             flag=0;    
       if (flag == 1)
        
            printf("%d",result[i]);
    }
    printf("\n");
    return 0;
}